- Create points A, B (used for radius)
- Create point C (this is where your arc will be centered
- create another point D (this will give the direction of your arc from C)
- construct a circle of radius AB centered on C
- construct ray CD
- construct perpendicular of CD through D, points F and G are intersections with
the circle
- construct arc with center C between F and G

Create new tool with inputs A, B, C, and D and output arc CFG!

To use it you click on your new tool icon and then poits A and B to give radius.
Then click C to specify the arc center and D to give it a direction. You can
slide D to make the arc bigger or smaller (from nothing to a semi-circle).
Then hide D (if you like) and you're done!

It works with GSP also. I had to create a segment AB to feed distance to the
"construct circle by center and radius." I also finally figured out the
selecting only the necessary inputs and outputs before "create new tool" gets
rid of all the intermediate cruft. I don't remember seeing this in the
documentation I could find.

This material is based upon work supported by the
National Science Foundation under Grant DUE-0226284.
Any opinions, findings, and conclusions or recommendations
expressed in this material are those of the author(s)
and do not necessarily reflect the views of the
National Science Foundation.